The present invention relates to a strain sensor adapted for a load cell weighing instrument or the like.
A conventional load cell weighing instrument has a beam body including a strain generating section, an insulating film formed of an inorganic oxide such as a silicon dioxide (SiO.sub.2), an aluminum trioxide (Al.sub.2 O.sub.3) or a tantalum pentaoxide (Ta.sub.2 O.sub.5) formed on the beam body, and thin film resistors formed to construct a strain gauge bridge circuit on the insulating film. When a load is, for example, applied to one end of the beam body, a strain is produced at the strain generating section, and this strain is transmitted to the thin film resistors through the insulating film. The resistances of the thin film resistors are varied in accordance with the magnitude of the strain, with the result that an output voltage from the bridge circuit will alter. If a predetermined load remains applied to the beam body, the output voltage of the bridge circuit will vary with time, and a so-called "creep phenomenon" will occur. The creep phenomenon is influenced by the quality of the material, the thickness, and the pattern of the thin film resistor, the quality of the material and the shape of the beam body, the quality of the material of the insulating film, and so forth.
FIG. 1 shows the creep characteristic of a conventional load cell which is constructed by forming a polyimide resin film of approx. 4 .mu.m thickness on the beam body made of a stainless steel (SUS630), and forming thin film resistors of approx. 1000 .ANG. thickness. When a rated load is applied to this load cell at a time t0, a rated output voltage V0 substantially corresponding to this load is almost instantaneously produced from this load cell. However, the output voltage of this load cell gradually increases. The output voltage of this load cell, for example, becomes (V0+.DELTA.V0) at a time t1 after 10 minutes. When the load is removed from the load cell at this time t1, the output voltage of the load cell will instantaneously drop to .DELTA.V1 having a value substantially equal to .DELTA.V0. Thereafter, the output voltage of this load cell will gradually drop and will substantially return to 0V at a time t2 after approx. 10 minutes has elapsed.
FIG. 2 shows the creep characteristic of another conventional load cell which employs a silicon dioxide film (SiO.sub.2) of 3 .mu.m thickness instead of the polyimide resin film of 4 .mu.m. In this case, the output voltage of this load cell drops to (V0-.DELTA.V2) 10 minutes after the rated load is applied to the load cell. When this load is removed from the load cell, the output voltage from this load cell drops from (V0-.DELTA.V2) to-.DELTA.V3, and is then gradually raised to 0V.
It is assumed that the input voltage VI of a load cell having the creep characteristic shown in FIG. 1 is 10,000 mV, the gauging factor K of the thin film resistors of this load cell is 1.8, and the strain E of the thin film resistors when a predetermined load is applied to this load cell (=.DELTA.L/L, where L represents the effective length of the thin film resistor, and .DELTA.L represents the variation of the effective length of the thin film resistor when the load is applied to the load cell) is 0.001. In this case, the output voltage V0 of this load cell is given by the following equation: ##EQU1##
In this case, the actually measured value of the variation .DELTA.V0 of the output voltage of the creep phenomenon was 20 .mu.V. Accordingly the creep becomes EQU .DELTA.V0/V0.times.100=20.times.10.sup.-3 /18.times.100=0.11 (%)
The accuracy of the load cell having a creep of 0.11 % is approx. 1/1000. When the influence of the temperature change is considered, an inaccurate load cell is the obvious result.
The creep of the load cell having the creep characteristic shown in FIG. 2 was: EQU -.DELTA.V2/V0.times.100=approx. -0.2%
Even in this case, the accuracy of the load cell becomes lower than 1/1000, and again, an inaccurate load cell is provided.
It has been heretofore considered that, in order to suppress the creep phenomena shown in FIGS. 1 and 2, the shape of a thin film resistor R formed on an insulating resin film as shown, for example, by the shaded part in FIG. 3 is altered. In other words, the creep characteristic can be adjusted by varying the ratio of the effective length L of this thin film resistor R to the width l of the sides of the thin film resistor R in FIG. 3. For instance, it is known that in order to smoothen the curve of the creep characteristic shown in FIG. 1 the size of the resistor R is so determined as to reduce the ratio l/L and in order to smoothen the curve of the creep characteristic shown in FIG. 2 that the size of the resistor R is so determined as to increase the ratio l/L. However, according to this method, the creep cannot be reduced to substantially 0, that is, to such an extent that the influence of the creeping phenomenon is negligible, and it is almost impossible to obtain a load cell of very high accuracy.